Abstract: Proposed one kind of numerical control 25Hz invertor anti-impact load’s protection strategy, through the computation equivalent output impedance’s method, played the effective protective function to the invertor. Has carried on the confirmation through the experiment, and has given the experiment profile.
Key word: Invertor; Impact load; Digital signal processor
Introduction
the 25Hz inversion power source is in the railroad sector signal generating plant, separately to rail return and minor loop power supply. Because it is in the entire railroad sector key equipment, thus in the voltage stability, the waveform distortion as well as the reliability has the very strict request. Specially in the reliability, it requests, in the load overloads, transformer turning on, the load to short-circuit in such impact situation not to create equipment’s damage, and requests to be able to withstand short-circuits for a long time such violent breakdown. But, because power semiconductor device’s overload capacity is bad, when the load short-circuits or turns on transformer such impact? Shoulders, will certainly to cause component’s damage. Therefore, docking enters when the shocking load carries on the protection is essential.
Based on the traditional simulation protection method’s insufficiency, this article proposed one kind comes through the computation equivalent output impedance to the impact load to carry on the corresponding protection the strategy, depends upon the DSP control the flexibility, can the short circuit, transformer turning on as well as the overload and so on carries on the varying degree the protection, and can withstand short-circuits for a long time such breakdown, the experiment proved that it is one kind of effective protection strategy.
1 tradition anti-load shock strategy
Single-phase invertor main circuit structure as shown in Figure 1, the control circuit basis output voltage and the inductance electric current, control the power component’s clear and the shutdown, after the output voltage passes through the first-level LC filter, obtained the high grade sine wave.
The traditional protection strategy has the following several kinds.
1.1 reduce the output voltage
The voltage dropping method is after examining the output excess current, immediately the voltage reference value pulls lowly, like this, then softened the surge current. What because electric circuit examination is the electric current spurt value, thus the electric circuit responds quickly, can carry on the effective protection to the bad current surge. When impact vanishing, the electric circuit can restore the normal output voltage voluntarily.
But, because the voltage dropping electric circuit is designs the good voltage dropping value beforehand, to adapt the big impact, must decide very much the setting value lowly, speaking of the mild overload situation, this kind of protection is excessive. But regarding short-circuits likely such impact, this method does not play the effective protective function.
1.2 join stagnates link’s spurt value current limiting electric circuit
This method is when examines the output current surpasses setting value I1, blocks switching valve’s gate extremely signal, the force electric current drops, when the electric current drops to another setting value I2, then removes the blockade signal, lets the electric circuit continue the normal work, when the electric current surpasses setting value I1 once more, then once more joins the blockade signal, is so redundant carries on. In fact this is through stagnates the output current the link comparator to control the blockade signal. The relative voltage dropping electric circuit says, it can act according to the different impact on cause the voltage to drop to the suitable degree, moreover, it may and so on attack the effective protective function greatly to the short circuit.
Because stagnates the link comparator’s clear and the blockade work in the high-frequency mode, therefore, this method will have the possibility to cause switching valve’s ultra-high frequency work, this regarding the switch component is quite dangerous, when appears short-circuits for a long time such breakdown, this method cannot to the equipment play the effective protective function.
2 DSP control short circuit protection strategy
Along with high speed figure processor DSP being published, the traditional analogue control is substituted gradually by the numerical control. As a result of the DSP easy debugging and nimble control, may cause the current limiting, the short circuit protection to do reliably, causes the system to be stabler.
2.1 systems to short circuit protection request and analysis
In order to guarantee that the 25Hz inversion power source’s reliability, the reasonable protection control needs to meet the following several requirements:
- - when overload cannot cause the output voltage to become zero, can only limit suitably;
- - when output short circuit can shut off the output voltage promptly, and can withstand the long-term short trouble;
- - works as when output turning on transformer, should not have the oversized influence to the output voltage;
- -, when overloads or the short trouble relieves, the equipment ought to be able the self-recovery work.
In the actual electric circuit, short-circuited the situation to be possible to be equal in has turned on a very heavy load, if speaking of the pure resistive load, namely load resistance R was small, thus, then might differentiate regarding the overload and the short circuit according to the R size, might come the correct judgment in the DSP procedure through the computation equivalent output impedance. Thus, regarding the above request, is must the output voltage coincidence pattern 2 curves then to be possible. In Figure 2, Vnom is the normal output voltage, Rnom amounts to the constant load. When overloads, the voltage drops suitably, but time load short circuit, limits the voltage to an extremely low value, assures equipment’s security.
2.2 control policies
Based on the above analysis, system when the overflow, the short circuit, the transformer turn on the electric circuit should have the corresponding protection, thus, DSP must distinguish to these 3 kind of situations, takes the different protective measures. Speaking of the sine wave output voltage, the overload situation can only be able to determine through an examination output current in cycle effective value, and, the short time’s overload is also the permission, should not have the influence to the output. But the load short-circuits such situation to request the invertor to be able to make the response rapidly. Based on this, the procedure in each switching period examination output magnitude of current, judges whether to short-circuit, but in each power frequency cycle computation output current’s effective value, the judgment outputs whether to overload.
DSP when each switching period interrupt carries on the sampling to the output current, when the output current is bigger than setting value Ilim, was the load short-circuits or the head end turns on the transformer, the procedure through has then limited switching valve’s gate extremely signal, guaranteed that the output current was smaller than Ilim. Because this time is the short circuit or the transformer turns on these two kind of situations regarding the load to be unable to differentiate, thus, when first several times examines the impact must restore the gate to try extremely again once more, after guarantees transformer turning on, does not output the big response. After carrying on several secondary load try, if the output current surpasses Ilim, is the situation which the load short-circuits, no longer again tries. What when turns on when is the transformer, is only instantaneous only then has the electric current overswing in turning on, after several secondary load try, the invertor again normal work, will not then have had the very tremendous influence to the output voltage.
In each power frequency cycle, routine calculation output current effective value, when output current’s effective value surpasses the rated value 120%, then belongs to the overload situation. When judges the overload the time to surpass the setting value (10s), then suitably reduces the output voltage according to the overload situation, the overload is more serious, the voltage drops is lower. Entire control policy’s flow as shown in Figure 3.
after 2.3 breakdowns relieve self-recovery
After the breakdown relieves, the system should restore normally in the shortest time. May judge the load correctly through the examination output current effective value and the output voltage effective value the situation. After the short trouble or the overload breakdown relieve, DSP obtains the load ratio of through the computation output voltage effective value and the electric current effective value the equivalent impedance, if of the discovery breakdown already relieved, then restores switching valve’s gate signal, the invertor then resumes the work extremely, but does not need to restore artificially.
3 experimental results
The above strategy obtained the confirmation on a 25Hz/1kVA inversion power source. The contravariant power source’s parameter is as follows: Input city electricity 220V/50Hz, the output voltage divides two groups, a group is the orbital power source, outputs 220V/25Hz, another group is the partial power source, outputs 110V/25Hz, two group voltage phase difference 90°, partially in advance track. This article take the partial voltage as an example, its output filter inductance is 4mH, outputs the filter electric capacity for 5μF. What the DSP controller uses is TI Corporation’s TMS320F241. The electric current uses the Hall component to carry on the sampling, raises the speed of response.
Figure 4 (a) to overload the 10s later voltage and the current waveform, after Figure 4 (b) is overload vanishing, the invertor rehabilitation profile. May see from Figure 4, when load overload, the output voltage is only reduces suitably, not completely cut-off output. Figure 5 (a) is when the load short circuit voltage and current waveform, when DSP examines the output current surpasses the setting value, then will immediately output the electricity point of fluidity to the security value, to adapt the situation which the transformer turns on, carried on 4 secondary load to try, might see from Figure 5, through the current limiting protection, the output current already limited below the equipment work security value, enabled the equipment to be able to withstand the long-term short circuit the situation. Figure 5 (b) is the short circuit excises the profile which the later output voltage electric current restores.
After Figure 6 is turns on the transformer, the output voltage profile, may see from the profile, the output voltage only had the short beat, can restore rapidly normally, will not have the bad influence to the entire output.
4 conclusions
This article proposes based on the DSP 25Hz inversion power source anti-impact load protection strategy, after the experiment proof is one good protection method, comes through the computation equivalent output impedance to the equipment to make the corresponding protection, may enable the system to withstand the bad breakdown situation, and can after the breakdown relieves restores voluntarily. This strategy may apply in generally invertor’s anti-impact protection.
